Research on remediation of cadmium-impacted soils: the role of PRB, the buffering capacity of rhamnosyl lipid fermentation solution, and the migration of cadmium-focused areas

To address the heavy metal "focusing effect", this study proposes a synergistic enhancement method combining rhamnolipid fermentation broth (RLs) and permeable reactive barrier (PRB). Through four groups of comparative experiments, the effects of RLs on soil electrical conductivity, electro-osmotic flow (EOF), cation exchange capacity (CEC), pH distribution, migration of the cadmium focusing zone, and cadmium speciation transformation were systematically analyzed. The results showed that RLs increased the negative zeta potential of soil surfaces (to -30 ∼ -41 mV) and electro-osmotic flow (by 24.5%), promoting the formation of mobile complexes between Cd and RLs, thereby effectively alleviating cadmium accumulation at the acid-base peak. PRB further immobilized migrating cadmium by regulating local pH (maintaining near-neutral conditions) and chemisorption on activated carbon. In the combined RLs-PRB group (EK3), the cadmium removal rate reached 80.77% in non-focusing zones (i.e., S1, S2, and S4, where no significant cadmium accumulation occurred between electrodes and PRB) in soil with an initial cadmium concentration 20 times higher than the regulatory limit. Meanwhile, soil pH remained neutral, CEC was stable, and energy consumption was significantly reduced. This study provides a theoretical basis and technical support for the practical application of the RLs-PRB synergistic electrokinetic remediation technology.